This invention relates to a multi-speed drive system for a portable lathe. Portable lathes typically have a stationary support ring attached to a mandrel which, in turn, is attached to the pipe or tube. A tool cutting holder ring is rotatably mounted on the stationary support ring and carries a tool holder in which is mounted a cutting tool. Several tool holders and tools may be affixed to a single cutting tool ring to perform multiple machining operations on the pipe end. Typically, such operations involve facing the end of the pipe and bevelling either the outer or inner periphery of the pipe adjacent the end.
A drive motor is mounted to the stationary support ring such that a pinion gear, attached to its rotating output shaft, engages a ring gear attached to the cutting tool holder ring. This serves to rotate the cutting tool ring with respect to the stationary support ring.
However, such portable lathes have been somewhat limited in their possible applications, since the drive mechanisms permitted only a single rotational speed of the cutting tool ring. The single speed drive systems inherently necessitated a compromise in the machining speed/torque parameters for machining different materials.
It is obviously desirable to be able to use a particular portable lathe on the widest variety of materials in order to minimize machining costs. However, when only a single speed drive is available, it is not possible to achieve a maximum machining speed and torque input for a wide variety of pipe materials. In machining harder pipe materials, such as stainless steel, the available torque input to the cutting tool ring may be below the optimum value and require a small cut for each revolution of the cutting tool ring. This, quite obviously, results in an inefficient operation, since it increases the time to achieve the desired results. If the portable lathe is used on a relatively soft material, such as cast iron, it is desirable to complete the machining in the shortest amount of time possible. This may require a higher rotational speed for the cutting tool ring than is available through the single speed drive mechanism.